Time variability in the H \(\alpha \)  flux (and in other lines and filters as well)
is a well-known phenomenon for CTTSs (e.g. [1]}, [2]}).
Monitoring campaigns of their accretion variability showed that this is typically dominated
by the timescale of rotational modulation of the accretion features (e.g. [3]}),
that is usually of the order of 0.5–2 weeks (e.g. [4]}).
If we exclude the most “extreme” cases (for instance, unstable accretors),
which can exhibit prominent variations over timescales of hours,
the amplitude of the \(\dot{M} \)  variability on CTTSs is typically observed to increase
from timescales of hours to timescales of days,
and then flatten out and remain approximately constant over timescales as long as years [3]}, [6]}.
This indicates that the global structure of accretion persists over hundreds of rotational cycles,
albeit with smaller-scale variations on shorter timescales (e.g. [7]}).
The typical \(\dot{M} \)  variability measured on week-long timescales
is \(\approx 0.4\) –0.5 dex [3]}, [9]},
out of which \(\approx 70~\%\)  can be explained in terms of geometric modulation of the accretion shock.
This implies that the intrinsic \(\dot{M} \)  variability on such timescales typically amounts to only \(\approx 0.15\)  dex.
Also, radiation-magnetohydrodynamical simulations of CTTSs (e.g. [10]}) indeed find that both stable and unstable accretors have stochastically changing line profiles, with paradoxically a more stationary appearance of the line profile for unstable accretors.
The estimate of [11]} for the variability of \(\dot{M} \)  onto the PDS 70 star,
0.56 dex over the rotation cycle, is indeed consistent with these typical estimates
of accretion variability for CTTSs. Time variability has also been seen in low-mass brown dwarfs,
for example in the \(\approx 47~M_{\mathrm {J}}\) ,
\(\approx 1\) -Myr-old brown dwarf candidate DENIS 1538–1038 [12]}.
